1. Field of the Invention
The present invention relates to a method of controlling the criticality of nuclear fuel cycle facilities, a method of producing uranium dioxide powders (UO2 powders) that are reactor fuels (UO2), a reactor fuel rod loaded in a nuclear reactor, and a fuel assembly. The present invention particularly relates to a method of controlling the criticality of a nuclear fuel cycle facility, such as a fuel fabrication facility or a fresh-fuel storage facility, using a reactor fuel rod fabricated using a UO2 powder containing less than 0.1% by weight of gadolinia (Gd2O3), to a method of producing a UO2 powder, to a reactor fuel rod, and to a fuel assembly. Furthermore, the present invention covers a method of controlling the criticality of a spent-fuel transport/storage cask or a fuel storage pool for storing the fuel assembly.
2. Related Art
In order to enhance the power uprating and operation period extension of nuclear power plants and in order to increase the economic efficiency thereof by suppressing the number of spent fuel assemblies in the future, the uranium enrichment of fuel is preferably increased. The increase in the uranium enrichment of fuel reduces the number of fresh fuel assemblies and the number of spent fuel assemblies per unit electricity generated and also greatly reduces fuel cycle costs.
Plants for fabricating fuel assemblies for commercial light water reactors are usually designed to pass a safety examination for the criticality safety of fuels with a uranium enrichment of up to 5% by weight. The safety examination is performed according to the guideline “KAKOU SHISETSU NO TAMENO ANZEN SHINSA SHISHIN (Safety Review Guideline for Uranium Processing Facility)”, whereby the construction of such plants is approved. Fuel storage pools and spent-fuel transport/storage casks are evaluated for criticality safety on the basis of the above concept.
Reactor fuels with a uranium enrichment of greater than 5% by weight (hereinafter referred to as “over -5% reactor fuels”) are strictly regulated under the guideline “TOKUTEI NO URAN KAKOU SHISETSU NO TAMENO ANZEN SHINSA SHISHIN (Safety Review guideline for Specific Uranium Processing Facility”.
In order to use the over -5% reactor fuels, design changes and/or equipment modifications are required for a fabricating step in view of criticality control. Design changes and/or equipment modifications are also required for a fresh-fuel transportation step, a fresh-fuel storage step, a spent-fuel transportation step, and a spent-fuel storage step. This may offset the reduction in fuel cycle costs due to the increase in the enrichment of reactor fuels.
For the fuel storage pools and the spent-fuel transport/storage casks, the handling of the following assemblies may be restricted because of criticality control, i.e., fuel assemblies including reactor fuel rods with a uranium enrichment of greater than 5% by weight or existing fuel assemblies with a maximum enrichment of 5% by weight or less. This may require equipment modifications.
In order to use the reactor fuels with a uranium enrichment of greater than 5% by weight, such design changes and/or equipment modifications required for each step cause an increase in cost, and therefore, may offset the reduction in fuel cycle costs due to the increase in the enrichment of reactor fuels as described above. Measures need to be taken against this problem. For the use of the reactor fuels with a uranium enrichment of greater than 5% by weight, the upper limit of the uranium enrichment of fuels for commercial light-water reactors is about 10% by weight for practical purposes.
The results of the investigation of such measures have shown that equipment modifications are required for steps handling uranium fuels, containing no burnable poison, with an enrichment of greater than 5% by weight in fuel fabrication facilities.
For the transportation and storage of fresh and spent fuel assemblies, the modification of transportation casks and transportation equipment may be avoided by making use of the reactivity-suppression effect (gadolinia credit) of a high concentration (several weight percent) of gadolinia, which is a burnable poison widely used for burnable poison-containing fuel assemblies.
Upon the implementation of the above measures, the type and concentration of a burnable poison added to reactor fuels are important.
Gadolinia, which is a burnable poison widely used for fuel rods for light-water reactors, has a large neutron absorption cross-section and high reactivity-suppression effect.
Erbium oxide (Er2O3) and boron (B) have a thermal neutron absorption cross-section less than that of gadolinium (Gd) and are effective in ensuring criticality safety in such a manner that a slight amount of erbium oxide is added to UO2 pellets as disclosed in Patent Document 1, or boron is used to coat the surfaces of UO2 pellets or the inner surfaces of fuel cladding tubes as disclosed in Patent Document 2. As shown in FIGS. 1 to 3, which are disclosed in Non-patent Document 1, Er-167, B-10, and Gd-157, which is an isotope of Gd, have a thermal neutron absorption cross-section of about 640, 3,840, and 254,080 barns, respectively, at room temperature (0.025 eV). That is, the thermal neutron absorption cross-sections of Er-167 and B-10 are far less than that of Gd-157.
If a burnable poison is added to a reactor fuel, the burnable poison remains in the reactor fuel at the end of an operation cycle depending on the type of the burnable poison and therefore may cause the reactivity loss of a reactor core. Hence, it is difficult to achieve the reduction in fuel cycle costs due to the increase in the enrichment of reactor fuels.
(Prior Art Documents Cited Above)
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-177241
Patent Document 2: Japanese Unexamined Patent Application Publication No. 4-212093
Non-patent Document 1: Nuclear Data Center at Japan Atomic Energy Agency, JENDL-3.3, [online], retrieved from the internet <URL: http://wwwndc.tokai-sc.jaea.go.jp/jendl/j33/J33_J.html>
Non-patent Document 2: Nuclear Materials Regulation Division/Nuclear Safety Bureau/Science and Technology Agency, “Nuclear Criticality Safety Handbook”, Nikkan-shobou, 1988
Non-patent Document 3: Thermal and Nuclear Power Engineering Society, “Atomic Fuel Cycle and Disposal Treatment”, 1986